1. Field of the Invention
This invention relates to moisture separators for use in the pressure vessel of a recirculating steam generator, and more particularly, to a centrifugal separator which is modular in design and has a low pressure drop.
2. Description of the Prior Art
Pressurized water nuclear reactors utilize a closed-cycle-system design principle. This means that radioactive coolant in the reactor cycle is completely separated from the turbine-generator cycle. The high-temperature coolant water from the reactor, however, must create steam for the turbine-generator cycle. The link connecting the two cycles is the steam generator. In one common design, the steam generator consists of two integral sections, an evaporator section and a steam drum section. The evaporator section consists of a U-tube heat exchanger, while the steam drum section houses moisture-separating equipment.
Gas-liquid or steam-water separators adaptable for use in the pressure vessel of a boiling water nuclear reactor steam generator have been described by J. T. Cochran et. al. in U.S. Pat. No. 3,216,182, by J. T. Cochran in U.S. Pat. No. 3,329,130, by C. H. Robbins et. al. in U.S. Pat. No. 3,603,062, by T. M. Modrak et al. in U.S. Pat. No. 3,788,282, by R. H. Moen et. al. in U.S. Pat. No. 3,902,876 and by W. R. Carson in U.S. Pat. No. 4,162,150. In such arrangements, a plurality of closely spaced separator units are mounted on a dome or cover atop a steam chamber above the nuclear reactor core. These primary separator units incorporate spiral vanes to impart a spinning action to the steam-water mixture and can be used in conjunction with secondary separator units.
The fundamental objective of the primary separator units is to remove most of the moisture from the steam-water mixture which exits the tube bundle of the steam generator. Because of the complexities of the two phase flow phenomenon in a separator, analytical predictions of whether a new primary separator design can perform according to these specifications have proven to be highly uncertain. It is therefore desirable to test new designs prior to their installation in a nuclear reactor. For economic reasons, it is preferable to test new designs in a steam water test loop, and to test them at full-scale steam flow rates, entrance qualities, and pressures. However, available separator test loops impose an upper bound on the size of a separator which can be tested at full scale. Even a separator test loop of the highest steam flow capabilities is insufficient to test full scale primary separators of the size commonly in use with large steam generators.
This problem has been solved by employing small-scale modular separators, which can be clustered together for use in a steam generator. In the steam generator, the modules function substantially independently of each other, and their total effect is additive. Thus, a new design can be tested at full scale using a single module.
However, while a small scale (e.g., 7 inch) modular separator if fully testable, the use of a cluster of small scale risers in a steam generator results in a decrease in riser area when compared to existing larger separators. This decrease in riser area results in a significant increase in pressure drop in the two-phase portion of the circulation loop, and tends to reduce the stability of the steam generator.
Therefore, what is needed is a modular primary separator which is small enough to be fully testable at full scale, but which has a low pressure drop comparable to that of large primary separators.